Two-electron reduction pathway

Under different conditions (in aqueous electrolyte) the selectivity of the cleavage reaction may be perturbed by the occurrence51-53 of a dimerization process. Thus, while the major process remains the two-electron reductive pathway, 20% of a dimer (y diketone) may be isolated from the cathodic reduction of PhC0CH2S02CH3. The absence of crosscoupling products when pairs of / -ketosulphones with different reduction potentials are reduced in a mixture may indicate that the dimerization is mainly a simple radical-radical coupling53 and not a nucleophilic substitution. 

However, as will be discussed below, this dismutation process is usually not the main pathway of H202 elimination in many living cells, where the two-electron reductive pathway is predominant. 

When water is used as a medium for CO2 reduction, it is important to suppress proton reduction to produce H2, which is more favorable in water than CO2 reduction. A hydrophobic polymer environment can provide such conditions by suppressing proton reduction to selectively carry out CO2 reduction. In an electrocatalytic CO2 reduction by CoPc confined in a poly(vinylpyridine) membrane coated on an electrode, selective CO2 reduction takes place, producing CO [119]. It is of further interest that in the electrocatalytic system, a third electron is injected into the Co(I)Pc(-3) - CO2 intermediate to produce CO and Co(I)Pc(-2) complex after the starting Co(II)Pc(-2) complex is reduced by two electrons to Co(I)Pc(-3) to form Co(I)Pc(-3) - CO2 with CO2. Such a mechanistic scheme can be represented by Fig. 20 [119]. Path I is the previously accepted scheme, and path II is the proposal of a new two-electron reduction pathway. Note also that for the CO2 reduction, a proton is also involved in an equilibrium process. It was inferred that in the polyvinylpyridine matrix, protonation and deprotonation takes place easily with the help of the pendant pyridine groups in a concerted fashion, resulting in favorable CO production (see also Fig. 20) [119]. 

An increase in the fraction of the four-electron reduction pathway at more reducing potentials (Fig. 18.10a, b) may be rationalized within at least two mechanisms. The first is based on the kinetic competition between the release of H2O2 from the ferric-hydroperoxo intermediate [Reaction (18.16) in Fig. 18.11] and its (reversible) reduction to a ferrous-hydroperoxo species, which undergoes rapid 0-0 bond heterolysis (18.13b). Because H2O2 and particularly HO2 are more basic ligands... 

Re(bpy)(CO)3Cl-modified electrodes has not yet been explained. However, from the cyclic voltammograms of fac-Re(bpy)(CO)3Cl (Fig. 14) and from the intermediate complexes formed by electrolysis in acetonitrile in the presence and absence of C02, two different electrocatalytic pathways (Fig. 15) were suggested144 initial one-electron reduction of the catalyst at ca. -1.5 V versus SCE followed by the reduction of C02 to give CO and C03, and initial two-electron reduction of the catalyst at ca. -1.8 V to give CO with no C03. The electrochemistry of [Re(CO)3(dmbpy)Cl] (dmbpy = 4,4 -dimethyl-2,2 -bipyridine) was investigated145 to obtain mechanistic information on C02 reduction, and the catalytic reac-... 

In contrast to Co-porphyrin complexes, the direct four-electron reduction of 02 has been only very rarely claimed to be catalyzed by a cobalt phthalocyanine 404 407 In particular cofadal binuclear Co-Pc complexes immobilized on pyrolytic graphite catalyze only the two-electron electroreduction of 02 to H202.408,409 However, recent work has established that an electropolymerized Co-Pc derivative provides a stable four-electron reduction pathway over a wide pH range 410... 

In the absence of nucleophile, the aryl halide undergoes a two-electron reductive cleavage according to an ECE-DISP mechanism (Scheme 2.21). The two-electron stoichiometry occurs because the aryl radical produced on the one-electron reductive cleavage is easier to reduce than the substrate. The competition between the ECE and DISP pathways is governed by the parameter... 

CuIV would allow the two-electron reduction of N20. Good electron-transfer pathways exist from the neighbouring CuA centre in the second subunit of the dimeric protein to Cu and CuIV, to allow rapid re-reduction of the Cuz centre. 

Several characterized NRPSs utilize alternative methods for chain termination. In some synthetases, the TE domain of the final module is replaced by an NAD(P)H-dependent reductase domain. Reduction of a peptidyl-S-PCP substrate through a two-electron reaction leads to the formation of a transient aldehyde, which is subsequently converted into a cyclic imine or hemiaminal through intramolecular cyclization. This two-electron reaction is utilized in the biosynthesis of nostocyclopeptides, the saframycins, ° and anthramycin. Alternatively, a four-electron reduction to the primary alcohol is observed in the biosynthesis of mycobacterial peptidolipids, linear gramicidin," " the myxalamides, lyngbyatoxin, " and myxochelin A 75,76 alternative four-electron reduction pathway involving aldehyde formation, transamination, and reduction to a primary amine occurs in the biosynthesis of myxochelin B. ... 

In accordance with these experimental results, Wang et al. employed density functional theory calculations to comprehensively examine the possible reduction pathways for EC molecules in super-molecular structures Li+—(EC) [n = 1—5) and found that, thermodynamically, both one- and two-electron reductive processes are possible.A complete array of the possible reduction products from EC was listed in their paper considering the various competitive pathways, and they concluded that both (CH2OCO2-Li)2 and (CH2CH20C02Li)2 are the leading species in SEI, while minority species such as lithium alkox-ide, lithium carbide, and the inorganic Li2C03 coexist. 

In Sch. 1, if the 002 radical anion reacts with CO2 in solution, CO is formed via a dimeric intermediate in which a C—O bond is formed. A second electron transfer to this intermediate from either the electrode or 002 leads to the formation of carhon monoxide. The ultimate result of this pathway is the formation of the two-electron reduction product, carbon monoxide, and carbonate. In this reaction, a second CO2 molecule acts as the acceptor of the oxide ion formed in the two-electron reduction of CO2 to CO. Many of the features of these reactions are common to the catalyzed reactions discussed in the following. Because free C02 is not present in the catalyzed two-electron reductions of CO2 to CO, the reduction potentials can be considerably less negative than that required to form CO2-. 

Several organocobalt species have been observed to catalyze the two-electron reduction of carbon dioxide. Depending on the pathway for reduction that is followed, different products arise. If the reaction proceeds via rapid sequential uptake of two electrons, carbon monoxide is produced ... 

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

A third pathway for oxime formation is given by tautomerization of nitroso compounds possessing an a-hydrogen (equation 9). Such a process involves an intramolecular redox reaction, in which the nitrogen undergoes a formal two-electron reduction, while the a-carbon is oxidized. Kinetic analysis of this conversion, as performed with a set of a-substituted 2-nitroso-l-phenylethane compounds, has revealed sensitivity toward both the bulkiness of the substituents and the initial concentration of the nitroso dimers128. For instance, tautomerization of 2-nitroso-l-phenylpropane to 28 has been proposed to play a role in the metabolism of methamphetamine by fortified rat liver tissue129. 

The favored peroxide disproportionation pathway requires two two-electron steps, with an optimal two-electron reduction potential of +0.86 V [5],... 

Isse, A.A., De Giusti, A. and Gennaro, A. (2006c) One- versus two-electron reaction pathways in the electrocatalytic reduction of benzyl bromide at silver cathodes. Tetrahedron Lett. 47, 7735-7739. 

A necessary condition for this two-site model is that electron transfer not occur between the two sites under the reducing conditions. If electron transfer is rapid, the two different reduction pathways would lead to a single, stable product. Several circumstances could block intersite electron transfer. One possibility is that the two sites are physically separated by a protein domain that prevents rapid redox equilibration. An alternative possibility is that a structural rearrangement takes place when the lower potential site is reduced. This rearrangement then prevents the electron transfer to the higher potential site. Our EXAFS re-... 

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